Interesting video of two types of 15th century armor, and the men in them demonstrating their agility and various fighting styles.
I had the impression that armor basically turned you into a lumbering tank, rather than much of a warrior. But this seems to not be the case...
I can add a few things about chainmail on that note. Hardly an 'expert' opinion, but I was given an opportunity to walk around in one for almost two full days by a friend from historical reenactment society for Days of Czersk. 1. They make almost no noise, even if you are running in one. 2. Despite weighing even ten kilogrammes (especially the ones that go past your belt and/or have sleeves) they don't throw you off-balance and the mass of all that grab is almost unnoticeable. That said, after taking it off I felt much lighter that I suspected I would be. 3. They act almost like heat-sinks on a hot day. I was sweating less in chainmail and tunic than in t-shirt and jeans on a sunny summer day.
Same source as my above chainmail post: weight can vary quite a lot, I've seen examples of longswords ranging from 0.8kg (yeah, I was also surprised) to 2.5kg despite being of comparable length (plus/minus 4cm), it's more about how the sword is balanced. Really good, properly weighted longsword can at the upper boundary of the above range and handle like it's almost weightless. Poorly weighted one can be under a kilogram and physically exert you.
I think it's a case of heraldry and Hollywood. This notion appears to have originated during the late nineteenth century as a joke. It entered popular fiction during the following decades, and the image was finally immortalized in 1944 when Sir Laurence Olivier used it in his movie Henry V—despite the protestations of his historical advisors, who included the eminent authority Sir James Mann, Master of the Armouries at HM Tower of London. Worth noting - infantry armor and cavalry armor ain't the same and that which will allow you to survive being hit by a lance is different from that which will allow you to survive the battle of Agincourt.There are a few exceptional instances when armor was extremely heavy or did indeed render its wearer almost “locked” in a certain position, such as armor for certain types of tournaments. Tournament armor was made for very specific occasions and would have been worn only for limited periods of time. The man-at-arms would have mounted his steed with the aid of his squire or a small step, and the last pieces of his armor could then be donned after securely sitting in the saddle.